Digital data transmission systems employing differentially encoded phase-shift keying (PSK) are well known in the art. Present day PSK systems generally employ hybrid arrangements of digital and analog circuit components. With the advent of relatively low-cost integrated circuit elements it has become possible to accomplish modulating and demodulating functions in data transmission apparatus with greater reliability and precision than with analog elements known to the prior art.
In differentially encoded PSK data transmission systems, as the name implies, data bits (taken one, two or more at a time) are represented by discrete phase differences measured between succeeding baud or symbol intervals. Decoding is then accomplished broadly by storing or delaying the absolute phases of received signals between baud intervals and subtracting the delayed phase from the present phase. The comparison between succeeding discrete phases can also be accomplished by correlating polarity samples of the received signal with those of a replica of itself delayed by an amount comparable to a baud interval. The latter correlation detection arrangement has been found to be highly adaptable to digital implementation. However, the signal-processing logic arrangements heretofore proposed have been unduly complex and generally have involved pluralities of up-down counters, auxiliary shift registers and digital logic circuits.
It is accordingly an object of this invention to provide an improved digital demodulator for PSK data signals.
It is another object of this invention to simplify the signal processing required with digital demodulators for PSK data signals employing correlation detection.